This invention relates to a control system for an induction motor driven by an inverter type for use, for example, in an elevator and, more particularly, to a reduction in the noise of an induction motor to control the speed of the motor.
Recently, the rotating speed of an induction motor can be controlled in the same degree as a d.c. motor due to the improvement in the control technique as an electronic technique has been developed. There is a variable voltage variable frequency controller (hereinbelow termed a VVVF controller) to control the speed of an induction motor. The VVVF controller converts an a.c. voltage into a d.c. voltage and again converts the converted d.c. voltage to an a.c. voltage by an inverter to vary the frequency of the voltage and to vary an output voltage inversely proportional to the frequency, thereby controlling the rotating speed of the induction motor.
Such a VVVF controller employs a pulse-amplitude modulation (hereinbelow termed PAM) control for variably controlling a d.c. voltage of the peak value of the output voltage of an inverter and pulse-width modulation (hereinbelow termed PWM) control for controlling a mean voltage by varying the time width by fixing the peak value.
When it is necessary to control the speed of an induction motor over a wide range from a stopping state to its full speed, for example, in an elevator, both the PAM control and the PWM control are employed because the noise of the induction motor increases if a full voltage is outputted from a converter to the motor at a lower speed.
FIG. 1 shows the constitution of a control system for an induction motor in a conventional elevator. Reference numeral 1 denotes a VVVF controller connected to a 3-phase a.c. power source. Numeral 2 denotes a converter having thyristors 2a to 2f connected in a bridge for converting an a.c. voltage supplied to the VVVF controller 1 to a d.c. voltage, numeral 3 denotes a regenerative converter having thyristors 3a to 3f connected in a bridge for converting a d.c. voltage from converter 2 to an a.c. voltage, numeral 4 denotes an autotransformer for transforming an a.c. voltage from the regenerative converter 3 for supplying it to the input terminal of the converter 2, numeral 5 denotes a reactor connected between the input terminals of the regenerative converter 3, numeral 6 denotes a smoothing condenser connected between the d.c. output terminals of the converters 2 and 3, and numeral 7 denotes a transistor inverter having transistors 7a to 7f and diodes 7g to 7l connected in a bridge for converting a d.c. voltage outputted from the converter 2 to a variable voltage variable frequency a.c. voltage. Numeral 8 denotes an induction motor driven by the output voltage of the transistor inverter 7 for an elevator, numeral 9 denotes a tachometer generator for detecting the rotating speed of the induction motor 8, numeral 10 denotes a sheave driven by the rotation of the induction motor 8, numeral 11 denotes a mechanical brake, numeral 12 denoes a deflection wheel, numeral 13 denotes a hoisting rope engaged between the sheave 10 and the deflection wheel 12, numeral 14 denotes an elevator cage attached to one end of the hoisting rop 13, numeral 15 denotes a balance weight attached to the other end of the hoisting rope 13, numeral 16 denotes a compensating rope connected through a balance wheel 17 to between the cage 14 and the balance weight 15, and numeral 18 denotes a governor rope engaged between a governor 19 and a tension wheel 20, and part of the governor rope 18 is fixed to the cage 14.
Numeral 21 denotes a pulse detector for detecting the rotation of the governor 19, and numeral 22 denotes control means for controlling the converter 2, the regenerative converter 3, and the transistor inverter 7 upon reception of signals from the tachometer generator 9 and the pulse detector 21. The control computer 22 has a CPU 22a, an interface 22b, a ROM 22c and a RAM 22d. Numerals 23 and 24 denote current transformers provided at the input and output sides of the VVVF controller, and numeral 25 denotes an a.c. reactor connected to the a.c. power source.
In the control system for the induction motor of the elevator constructed as described above, the 3-phase a.c. voltages supplied from the 3-phase a.c. power source are converted by the converter 2 to a d.c. voltage, which is, in turn, smoothed by the condenser 6, supplied to the transistor inverter 7, which again converts it to an arbitrary voltage and frequency a.c. voltage. The transistor inverter 7 is PWM-controlled by the control computer 22.
The PWM-control, as shown in FIG. 2(a), compares the triangular modulation voltage 40 with control voltages 41 and 42 from the phases, such as phases R and S of the 3-phase a.c. power source inputted to the control computer 22 to gate the transistors 7a to 7f of the transistor inverter 7, thereby producing an approximately sinusoidal wave. FIG. 2(b) shows an example of the voltage V.sub.U of the phase U outputted from the transistor inverter 7, and FIG. 2(c) shows an example of the voltage V.sub.V of the phase V, and FIG. 2(d) shows the voltage V.sub.UV between the lines of the phases U and V.
The frequency of the modulated voltage 40 used for the PWM control ordinarily employs a frequency of approximately 1 KHz (hereinbelow termed "carrier frequency"). The induction motor 8 generates noise due to this carrier frequency.
In an elevator, if a living room is disposed adjacent to a machine room, noise reduction is required. Thus, a large a.c. reactor is inserted on the input side of the induction motor 8 to attenuate the high frequency, or a special low noise motor is used as the induction motor 8. However, both have demerits of large size and expensive cost. There is generally a method of increasing the carrier frequency to approximately 10 KHz which is scarcely heard by ears, but the switching of the transistor inverter 7 results in an increase in the switching loss. Thus, the transistor capacity of the transistor inverter 7 must be increased to as to eliminate a thermal problem caused thereby, thus increasing the cost.
In the VVVF controller in FIG. 1, the phase angle of the power source voltage is controlled by PAM control of the converter 2 or the regenerative converter 3 when a large current flows to the induction motor at the time of accelerating the elevator for lowering the d.c. voltage as compared with that at a rated speed, thereby reducing the noise of the induction motor 8.
However, when the phase angle is controlled by the thyristors of the converter 2 or the regenerative converter 3, a number of harmonic waves of fifth and seventh orders are generated as heretofore known. Since the inverter or CVCF for an equipment, such as an air conditioner, which generate such harmonic waves have increased recently in building, there is a trend of restricting the harmonic waves generated by various equipment.